Add table of built-in cosmologies, other minor documentation cleanups.

* closes #1147
Fix sphinx warning, add note about doc update

* Extra space before code block in Built-in Cosmologies section
  of index.rst
* Add warning to parameters.py to update the documentation table
  when adding a new cosmology
Make sure the list of modules being iterated over cannot change during the iteration.
Merge pull request #1527 from aconley/cosmo_builtin_docs

Add table of built-in cosmologies, other minor documentation cleanups.
Merge pull request #1533 from astrofrog/fix-catch-warning-issue

Make sure the list of modules being iterated over cannot change during iteration
Change interpretation of lists in Table.__getitem__ (closes #274)
Duck type when item in __getitem__ is not one of a few specified possibilities
Add tests and documentation for getting sub-set of columns with list of names